Oh, I love talking about this sh1t
I guess the biggest factor is the sound you are going for. I'm a synth guy, I more than enough hardware synths, some VSTi's and there are synths in the MPC I lean on a lot. I have set up Banks on my Novation Launch Control for the OPX4, MiniD, and Jura, and the more a study these, the more unique sounds I get (I need to set up the Odyssey, but I've yet to come up with a plan to map that), their flexibility, and adaptation to subtle changes, make them a long term investment in time and more valuble to me. And being integrated into the MPC is such a bonus. But I don't touch the other synths, apart from Electric and Drumsynth occasionally. I love reaching for the synth parameters to make changes makes things sit and feel differently, before reaching for effects to make things sound better.
If it's a synth sound you are after, I would always get a softsynth that models a synthesizer over a sample of an of a synthesizer. And I've always got enough to play with to wait for Black Friday or other opportunity's. I've stopped buying VST effects for my DAW, but the clipper will get picked up next time on sale. I rarely use presets on a synth, and a sample of a synth is more baked in, it's already not your sound before you 've started.
For percussion of real sounds, I have stacks of options in VSTi's (Nearly all of Addictive Drums, and XO, Shimmer/Shake/Strike, some Latin set ups for Kontact). So I'd prefer to create my own loops. for the MPC, with all the tools my DAW has. These VSTi's have better dynamics and more options than the MPC sample sets. But if I didn't have these, I think sampled percussion and kits are probably a good shout for an expansion pack.
For real world sounding instruments, I just don't see the dynamics and and authenticity being available in the MPC. This is fine for lot's a genres, If you're just looking for some chord comping a piano expansion is probably a good choice. But again I have other good options and would rather create in a VST in my DAW and sample as a loop, than come up with in the MPC. And there are so many free options available now, like Spite Fire Labs
https://labs.spitfireaudio.com/Then there's acoustic modelling (Electric in the MPC kid of thing). I would love Pianoteq for pianos, keys, vibes over sampled options. I have Samplemodelling's Brass Audio Modelling's Cello (I will expand this some day), and and I think this is more my preferred way to get real sounding sounds than stacks of samples trying to get the same realism with less flexibilty. I have Chromophone, if Objekt ever became available as a VSTi I would be all over that.
So yeah, always synths over samples for me. Sample the sounds you make and find, leave the synthesizers to synthesis. I'd rather invest in something with longevity, than keep buying lots of a small treats, that fall short eventually.